For creating a 3D image, the term “structured light” means that a camera is applied together with a projector. The projector superimposes a pattern onto the scene to be recorded, and the 3D shape of the scene is determined from the geometrical analysis of this pattern. Both the camera and the projector have their own frame rates F1, F2. The projector emits the pattern at the frame rate F2 while the camera or image sensor, respectively, record individual frames, that is, expose the pixels of the image sensor, at the frame rate F1. It is important for the geometrical analysis that the respective image recorded by the camera is superimposed with the desired pattern for as long a portion of the exposure time as possible. This again requires that projector and camera are synchronized such that at least the phase response of the two frame rates F1, F2 is controlled, that is, adjusted. In this case, the two frame rates F1, F2 should at best be equal in magnitude. If not, the frame rates F1, F2 should be matched accordingly. The condition of an equal phase response would also be met if these rates were at least integer multiples of one another.
The use of non-synchronized cameras is called “free running”. In this operating mode, even low-cost cameras can produce a frame rate of approx. 60 Hz up to 69 Hz, and the subsequent image readout can be performed while exposing the next frame.
But if such cameras are used in triggered operation, the frame rate drops down to 20 Hz to 30 Hz. This is due to the architecture of the processor because it first has to process exposure and then the subsequent reading out of the image before another exposure will be possible. In order to retain the original frame rate, the processor would have to be able to process the trigger signals from the projector regardless of the readout process and synchronously with exposure, which is called pipeline triggering. Processors of low-cost cameras cannot do this.
Cameras that can, for example, deliver 60 Hz in synchronized operation, that is, cameras that comprise a pipeline triggering mode, are two to three times more expensive than low-cost cameras.
A method for synchronized operation of a camera is known from the application report titled “Using the DLP Pico 2.0 Kit for Structured Light Applications” by Texas Instruments. As can be seen on page 12 there, the projector (pattern) provides a synchronization pulse (sync pulse). The synchronization pulse comprises a variable delay (sync delay). In this way, the starting point of the synchronization pulse can be changed.
The phases of the two frame rates can therefore be synchronized. However, this requires that the two frame rates are equal in size or integer multiples of one another.
DE 696 28 956 T2 discloses a method for synchronizing the recording of images by a camera and the generation of data that determine the position of the camera in a system for scanning an object and for generating data that determine points in a three-dimensional space that match the points on the surface of the object. The above method includes a scanning means that is mounted onto a multiply jointed arm for being moved by an operator and that can be used to scan an object and capture data from multiple points on the surface of the object, wherein the scanning means comprises a laser for emitting laser light onto the surface of the object and a camera for capturing the laser light reflected from the surface of the object by recording images at instants determined by a synchronization signal, said method comprising the following steps: receiving the synchronization signal for the camera, which determines the image recording instants of the camera, and the subsequent generation and output of trigger pulses to the position calculation means.
A device for three-dimensional optical measuring of objects with a topometric method of measurement in which images of projection patterns projected onto an object are recorded and evaluated is known from DE 10 2006 002 077 A1. The device includes a projector with a light source, an image recording unit and an image evaluation unit. The light source of the projector is a light arc lamp, and the image recording unit is triggered as a function of the intensity of the light arc lamp. As an alternative, the patent specification describes the operation of the recording unit in non-triggered mode, i.e. in free running mode. In this case, the light arc lamp or its current characteristics are triggered.